In order to print the large quantities of information issuing from modern computer systems it has been necessary to develop typewriting machines capable of operating at extraordinarily high speeds. One of the most successful types of such machines is the dot matrix printer in which a printing head contains a plurality of electrically actuated wires capable of producing a matrix of dots to form the characters. A significant advantage of this type of machine is that a single printing head can be used to print an endless number of letters and characters in a variety of languages such as, for example, English and Chinese.
U.S. Pat. No. 4,389,128, granted June 21, 1983 to K. Asano et al for "Print Head for a Dot Matrix Printer" discloses what can be termed a state-of-the-art printer head of the type with which the present invention is concerned. The Asano et al print head has a plurality of radially arranged wire actuating armatures which are maintained in a non-print position by a magnetic structure including an annular permanent magnet. The individual armatures are spring biased to move to their print positions when individual electromagnets associated with the magnetic structure are energized to cancel out the flux from the permanent magnet acting on the armatures. The electromagnets are arranged in a circular array within the annular permanent magnet. With this arrangement, when the overall size, i.e. diameter, of the head is held to a minumum the electromagnets are disposed in a cramped space with the result that the electromagnet coils are difficult to manufacture and heat dissipation from the coils is a difficult problem as is explained in the Asano et al patent. If the print head is made larger to overcome the space and heat problems an excessively large permanent magnet must be used. Moreover, for very high speed printing the size and weight of the printer head must be reduced to the absolute minimum, so making the head larger is counterproductive.
A somewhat simpler printer head is disclosed in U.S. Pat. No. 4,348,120, granted Sept. 7, 1982 to M. Isobe et al for "Printing Head for a Dot Printer". The arrangement of components there disclosed is essentially the same as in the printer head of the Asano et al patent so the Isobe et al printer head possesses the same shortcomings.
Other arrangements for the components of a printer head are disclosed in the following U.S. Pat. Nos.:
4,222,674, granted Sept. 16, 1980 to M. Mori et al for "Head Portion of a Dot Printer";
4,225,250, granted Sept. 30, 1980 to R. E. Wagner et al for "Segmented-Ring Magnet Print Head"; and
4,230,412, granted Oct. 28, 1980 to D. G. Herbert for "Matrix Print Head Assembly".
Notwithstanding the efforts of these prior inventors and others there continues to be a need for a compact, light weight, reliable printer head which can be manufactured inexpensively.